STRUCTURE OF MATERIALS

The make up of an atom The simple idea of an atom is that an atom has a nucleus and negatively charged electrons whirling around the nucleus. Within the nucleus, there are protons (positively charged) and neutrons (no charge) Electrostatic forces hold the nucleus and the electrons together.

Three types of atomic bonds: Covalent bonds Ionic bonds Metallic bonds

Covalent bonds Known as primary bonding. A pair of atoms are Shared by two or more elements.

Ionic bonds Cations (+) and anions (-) are attracted to each other in an ionic bond. Electrons may be transferred from one atom to another in ionisation.

Metallic bond (+) Cations in a ‘sea’ of (-) electrons. The movement of these electrons makes metals good conductors of heat and electricity.

Crystalline structures Body-centred cubic (BCC) Face-centred cubic (FCC) Close-packed hexagonal (CPH)

Slip in BCC and FCC structures As atoms in FCC are more closely packed than BCC, slip will occur more easily. FCC metals are ductile and BCC metals are more brittle.

Properties of metals Conduct heat and electricity Malleable and ductile Electron donors (form oxides) Can take a shine High density High tensile strength Solid at room temperature (except Mercury)

Crystal defects There are two categories of defects in a crystal Line defects -Dislocations. Point defects -Vacancy -Substitution -Interstitial

Vacancy point defect If there is an atom missing from the lattice, then the whole lattice is distorted as other atoms are forced into the vacant space.

Substitution point defect In this case, a much larger atom has been substituted in the lattice and distorts the structure.

Interstitial point defect In this case, a foreign atom has moved into the space between the atoms of the lattice.